Analysis and Design of Elevated Water Tank with Frame and Shaft Type Tappered Staging

Abstract

Water is the prime necessity for survival. Liquid storage tanks are used extensively by municipalities and industries for storing water, inflammable liquids and other chemicals. Thus Water tanks are very important for public utility and for industrial structure. Selection of form for a water-tank and staging is a very important step in the design. Intze type containers have been used from 200 to 1200 m3 capacity with frame or shaft type vertical or tappered staging. Elevated tank, which typically consist of a large mass supported on the top of a slender staging, are particularly susceptible to earthquake damage. Thus seismic behaviour of elevated tanks should be known and understood. This report presents a comprehensive analytical investigation concerning behavior and designing of Elevated Water tank with Frame and Shaft type Tappered Staging subjected to static and dynamic loading with special emphasis on IS: 11682, GSDMA guide line. Different technique of modeling along with process of modeling for different components in SAP2000 software has been discussed. Step-by-Step procedure of modeling in SAP2000 software by considering two mass model as per GSDMA guide line has also been covered. In addition to the vertical loads, Frame staging is also subjected to horizontal forces. Both these forces produce axial tension or compression in columns as well as moment and shear force in column section. The effects of No of column on staging, Tappering of staging and change in aspect ratio of staging diameter on internal forces of staging are reviewed in the analysis, both for Tank full and tank empty condition. On basis of that an attempt is made to find out optimum diameter of staging considering 'No- Tension' in column. Similarly, lateral load also affect stability of foundation. Here attempt is made to find out optimum diameter of annular raft footing by fulfilling the stability requirements including 'No-Uplift'. Similarly, shaft staging is also subjected to horizontal forces in addition to vertical loads. Both these forces produce axial tension or compression as well as moment and shear force in shaft section. The effects of Tappering of shaft and Change in aspect ratio of shaft diameter on internal forces of shaft are reviewed and considered in the Analysis, both for Tank full and tank empty condition. On basis of that an attempt is made to find out optimum diameter of shaft based on 'Minimum-Tension'in section. Excessive liquid sloshing may cause the structural failure, so response of sloshing fluid on displacement with frame and shaft type staging needs to be study. Time history analysis with direct integration method is used for carry out dynamic analysis. Time history data of Uttarkashi earthquake and Chamoli earthquake has been used. A detailed Parametric studies has been carried out,to study the effects of No of column in Frame Staging, Batter of staging, Change in aspect ratio of staging diameter and Capacity of tank on Stiffness, Time Period, Displacement and Internal Forces of Shaft and Frame type staging. Similary all above parametric studies has been performed by using two different methods of analysis namely static and dynamic-time hisotry to study the behaviour of staging. An attempt is made, by preparing excel sheet for designing Intze tank with Frame and Shaft type staging. Designing of Annular raft footing for frame and shaft type staging. An excel sheet has also been made to found out lateral load like Wind and Earthquake using IS:875 (Part-3)-1987 and GSDMA guide line respectively. Conclusions and Future scope of work are included in final chapter. Few of the conclusion are as follows. _ Envelope of worst load combination shows axial tension in 250m3 tank column, while staging of 500m3 and 1000m3 shows axial compression in column. _ Optimum dimension of staging for 250m3 tank are 70% (6.28m) with 60 inclination or 80% (8.04m) with 00 inclination, by considering no tension in column as well as fulfill all stability requirement of foundation. _ Envelope of worst load combination shows axial tension in shaft is unavoidable even change the parameters like Increase the capacity of tank from 250m3 to 1000m3 or Increase the inclination of shaft from 00 to 60 degree or Increase the diameter of shaft from 60% to 80%, however value of axial tension is reduced by increase in the diameter as well as tapering of shaft section. _ Displacement values for same earthquake zone-IV shows different value in Uttarkashi and Chamoli earthquake than code value. For the same zone displacement value obtained by code is much lesser than Dynamic analysis of actual earthquake value. _ Two different Dynamic analysis namely Response spectrum and Time History shows Base shear values very close. _ For comparing the effect of two different capacity of tank for sloshing displacement a 6-column frame staging with 250m3 and 500m3 tank selected, which shows 16.4% higher displacement as compared to 250m3 tank. So it can be concluded capacity of tank affect response of sloshing displacement. _ Base shear obtained by static analysis using GSDMA guide line and dynamic analysis using Uttarkashi earthquake are very close and comparable.